1. Field of Art
This invention relates to polypropylene compositions reinforced by glass fibers. More particularly, it relates to polypropylene compositions reinforced by glass fibers having superiority in mechanical properties, rigidity and visual appearance.
2. Prior Arts
Recently, demand of high rigidity and high mechanical strength for thermoplastic resins used as raw materials for parts of automobile, electric appliances and various kinds of other industrial products is increasing. In compliance with this demand, in case of polypropylene (hereinafter referred to as PP) also, development of polypropylene reinforced by glass fibers (hereinafter referred to as FR-PP) which is obtained by addition of glass fibers to PP is going on actively.
However, as PP is a non-polar resin, wetting (or affinity)of this PP to the glass fibers on the surface of these glass fibers is worse and even when glass fibers are simply added to PP, improvement effectiveness is hardly recognizable in mechanical strength (such as tensile strength, flexural strength or the like).
On this account, PP modified with unsaturated carboxylic acid or anhydride thereof (hereinafter referred to as unsaturated carboxylic acid group) (hereinafter referred to modified PP) is added to PP and as glass fibers, those in which the surfaces are treated with a silane coupling agent having reactivity with unsaturated carboxylic acid group are used, as a method for improving wetting property, i.e. adhesiveness between PP and glass fibers.
On the other hand, with the increase of adhesiveness between the PP and glass fibers, there is a tendency that visual appearance of molded articles becomes worse.
Further, the improvement of rigidity can be attained by increasing the amount of to-be-added glass fibers but there is a problem that the increase of addition of glass fibers makes worse the visual appearance of molded articles. Moreover, if the amount of addition of glass fibers is increased, besides the deteriration of the above-mentioned visual appearance, the increase of specific gravity of molded articles loses the merit of lightweightness and thus there is a limit in the increase of addition amount of glass fibers.
Accordingly, it is desirable to obtain high rigidity, high mechanical strength as well as good visual appearance with a small amount of addition of glass fibers.
On this account, there is known a method in which a polyamide resin, particularly nylon 6 or nylon 66, and glass fibers are added to PP. In this case, in order to improve the compatibility of PP with a polyamide resin the addition of modified PP to PP is carried out. However, in case of compositions consisting of PP, modified PP, a common polyamide resin such as nylon 6, nylon 66 and glass fibers, there is a problem that mechanical strength, and rigidity are exceedingly reduced on account of moisture absorption when they are made into molded articles because nylon 6 and nylon 66 have moisture absorption characteristics.
Further in such a system when heating is carried out for the purpose of melting and kneading, violent reaction takes place between carboxylic acid radical derived from the above-mentioned modified PP and a polyamide resin, formed molded articles not only turns their color to yellowish shade greatly but also lose their lustre remarkably. As the result, a problem occurs in the point that visual appearance shows a surface state of delustered and extremely uneven in lustre and nice appearance cannot be obtained.
The inventors of the present invention made a great deal of studies in order to solve the above-mentioned problems of FR-PP. As the result, they used a specified polyamide resin obtained by the polycondensation of metaxylene and adipic acid, as a polyamide resin, mixing a modified PP or PP containing modified PP and glass fibers therewith followed by melting and kneading, and found that thus obtained polypropylene composition reinforced by glass fibers improved the mechanical strength and rigidity of molded articles and showed superior visual appearance. Thus they completed the present invention.
As evident from the foregoing description, an object of the present invention is to provide a polypropylene composition reinforced by glass fibers, from which molded articles having improved mechanical strength and rigidity and excellent visual appearance can be made.
The object of the present invention can be attained according to the compositions of the present invention having following constitutional elements.